1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to an alkali metal electrochemical cell having reduced voltage delay and reduced irreversible or permanent Rdc growth. A preferred couple is a lithium/silver vanadium oxide (Li/SVO) cell. In such cells, it is desirable to reduce voltage delay and irreversible Rdc growth at about 25% to 70% depth-of-discharge (DOD), where these phenomena typically occur.
2. Prior Art
Voltage delay is a phenomenon typically exhibited in a lithium/silver vanadium oxide cell that has been depleted of about 25% to 70% of its capacity and is subjected to high current pulse discharge applications. In other portions of the discharge curve for a Li/SVO cell, Rdc, which is caused by a passivation layer buildup on the anode surface, is substantially diminished, if not completely eliminated, by pulse discharging the cell to remove the passivation layer.
The voltage response of a cell that does not exhibit voltage delay during the application of a short duration pulse or pulse train has a distinct signature. In particular, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse. FIG. 1 is a graph showing an illustrative discharge curve 10 as a typical or “ideal” waveform of a cell during the application of a series of pulses as a pulse train that does not exhibit voltage delay.
On the other hand, the voltage response of a cell that exhibits voltage delay during the application of a short duration pulse or during a pulse train can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the first pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied. FIG. 2 is a graph showing an illustrative discharge curve 12 as the voltage waveform of a cell that exhibits both forms of voltage delay.
The initial drop in cell potential during the application of a short duration pulse reflects the resistance of the cell, i.e., the resistance due to the cathode, the cathode-electrolyte interphase, the anode, and the anode-electrolyte interphase. The formation of a passivating surface film is unavoidable for alkali metal, and in particular, lithium metal anodes due to their relatively low potential and high reactivity towards organic electrolytes. In the absence of voltage delay, the resistance due to passivated films on the anode and/or cathode is negligible. Thus, the ideal anode surface film should be electrically insulating and ionically conducting. While most alkali metal, and in particular, lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. In the event of voltage delay, the resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation that often results in reduced discharge voltage and reduced cell capacity. In other words, the drop in potential between the background voltage and the lowest voltage under pulse discharge conditions, excluding voltage delay, is an indication of the conductivity of the cell, i.e., the conductivity of the cathode, anode, electrolyte, and surface films, while the gradual decrease in cell potential during the application of the pulse train is due to the polarization of the electrodes and electrolyte.
The anodes of electrolytic capacitors can develop microfractures after extended periods of non-use. It is known that reforming electrolytic capacitors at least partially restores and preserves their charging efficiency. An industry-recognized standard is to reform implantable capacitors by pulse discharging the connected electrochemical cell about once every three months throughout the useful life of the medical device. Pulse discharging also serves to break up and substantially dissipates the passivating surface film on the lithium anode. However, at about 25% DOD to about 70% DOD, this frequency of pulse discharging, while acceptable for capacitor reform, does not adequately reduce voltage delay and irreversible Rdc caused by the passivating surface film on the lithium anode below that which is acceptable.
Thus, the existence of voltage delay is an undesirable characteristic of Li/SVO cells subjected to current pulse discharge conditions in terms of its influence on devices such as medical devices including implantable pacemakers, cardiac defibrillators and automatic implantable cardioverter defibrillators. Depressed discharge voltages and voltage delay are undesirable because they may limit the effectiveness and even the proper functioning of both the cell and the associated electrically powered device under current pulse discharge conditions.